A heavy rock is lifted using a lever of length 80cm such that othe fulcrum is shifted 10cm away from the load then by what percentage the mechanical advantage changes?

You have not given the value of load arm .
Suppose,
Load arm=x cm
Effort arm=(80-x) cm
Then,
mechanical advantage, M=effort arm / load arm
=(80-x) / x
Now, if fulcrum is shifted 10 cm away from the load,
then load arm=x +10
Effort arm=[80-(x+10)]=70-x
Then mechanical advantage, M'=(70-x) / (x+10)
Put the value  of x and calculate M/M'
You will get the desired answer.